Bhadeshia H.K.D.H. Bainite In Steels. Transformations, Microstructure and Properties
Подождите немного. Документ загружается.
10 Acicular Ferrite
10.1 General Characteristics and Morphology
Highly organised microstructures can often be found in steels; for example,
ferrite plates frequently grow in the form of packets containing parallel plates
which are in the same crystallographic orientation. This can be harmful to
mechanical properties because cleavage cracks can propagate readily across
the packets.
Some of the most exciting recent developments in wrought and welded steel
technology have involved acicular ferrite (Grong and Matlock, 1986; Abson and
Pargeter, 1986). Far from being organised, this microstructure is better
described as chaotic. The plates of acicular ferrite nucleate heterogeneously
on small nonmetallic inclusions and radiate in many different directions
from these point nucleation sites (Fig. 10.1). Crystallographic data show highly
misoriented plates nucleated on the same inclusion (Gourgues et al:, 2000).
Propagating cracks are then de¯ected on each encounter with a differently
oriented acicular ferrite plate. This gives rise to superior mechanical properties,
especially toughness.
Acicular ferrite is therefore widely recognised to be a desirable microstruc-
ture. This chapter deals with the mechanism by which it forms and with the
role of inclusions in stimulating its formation.
The term acicular means shaped and pointed like a needle but this is
misleading because the true shape is that of a lenticular plate. The aspect
ratio of the plates has never been measured but in random planar sections,
they are typically about 10 mm long and approximately 1 mm wide, so that the
true aspect ratio is likely to be much smaller than 0.1.
An arc-weld deposit typically contains some 10
18
m
3
inclusions of a size
greater than 0.05 mm, with a mean size of about 0.4 mm, distributed throughout
the microstructure. Inclusions form as oxygen in the liquid weld metal reacts
with strong deoxidising elements such as silicon, aluminium and titanium.
Slag-forming compounds which form a part of the system designed to protect
weld metal from the environment, may also become trapped in the solid at the
advancing -ferrite/liquid interface. Inclusions promote intragranular
nucleation of acicular ferrite plates and hence improve toughness without
[13:35 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 237 237-276
237
compromising strength. But they also are responsible for the nucleation of
voids during ductile fracture, or the nucleation of cleavage cracks during
brittle fracture. Achieving a balance between these con¯icting factors is the
essence of good design. The inclusion microstructure is particularly important
in this respect (Fig. 10.2). For example, nonmetallic particles in certain sub-
merged arc weld deposits consist of titanium nitride cores, encapsulated in a
glassy phase containing manganese, silicon and aluminium oxides, with a thin
layer of manganese sulphide titanium oxide partly covering the surface of the
inclusions (Barbaro et al:, 1988). The development of this complex microstruc-
ture has been modelled using nucleation and growth theory (Babu et al:, 1995).
Inclusions can be oxides or other compounds but the important point is that
they may stimulate acicular ferrite (Ito and Nakanishi, 1976). The nucleation of
a single plate on an inclusion can in turn stimulate others to nucleate
autocatalytically, so that a one-to-one correspondence between the number
of active inclusions and the number of acicular ferrite plates is not expected
(Ricks et al:, 1982).
The shape change accompanying the growth of acicular ferrite has been
characterised qualitatively as an invariant-plane strain (Fig. 10.3). Like bainite,
the shape deformation causes plastic deformation in the adjacent austenite. The
Bainite in Steels
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 238 237-276
238
Fig. 10.1 Replica transmission electron micrograph of acicular ferrite plates in a
steel weld deposit (Barritte, 1982).
resulting dislocations are inherited by the acicular ferrite as it grows, giving a
dislocation density which is typically at 10
14
m
2
, and which contribute some
145 MPa to its strength. The stored energy of acicular ferrite is similar to that of
bainite at about 400 J mol
1
(Strangwood and Bhadeshia, 1987; Yang and
Bhadeshia, 1987). Consistent with the observed shape change, microanalysis
experiments prove that there is no long-range partitioning of substitutional
solutes during the formation of acicular ferrite (Strangwood, 1987); indeed,
atomic resolution experiments have demonstrated that there is no partitioning
on the ®nest conceivable scale (Chandrasekharaiah et al:, 1994).
Acicular ferrite clearly grows by a displacive mechanism so there are other
consequences on the development of microstructure. Thus, plates of acicular
ferrite are con®ned to the grains in which they grow because the coordinated
movement of atoms associated with the displacive transformation mechanism
cannot be sustained across grain boundaries. The
a
= orientation relationship
is always found to be one in which a close-packed plane of the austenite is
nearly parallel to the most densely packed plane of
a
. Corresponding close-
packed directions within these planes are found to be within a few degrees of
each other (Strangwood and Bhadeshia, 1987). As with bainite, the size of the
acicular ferrite plates increases with transformation temperature; Horii
Acicular Ferrite
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 239 237-276
239
Fig. 10.2 Scanning transmission electron micrograph of a nonmetallic inclusion in
a steel weld metal. The inclusion surface is very irregular, and it features many
phases (after Barritte, 1982).
et al:(1988) reported that the apparent plate thickness and length changed from
about 1 to 2 mm as the weld cooling rate was reduced.
10.2 Mechanism of Growth
The acicular ferrite transformation exhibits the incomplete-reaction phenom-
enon, an important characteristic of bainite. The extent of reaction decreases
towards zero as the transformation temperature is increased towards B
S
(Yang
and Bhadeshia, 1987; Strangwood and Bhadeshia, 1987). Isothermal transfor-
mation stops when the carbon concentration of the residual austenite exceeds
the T
0
0
curve (Fig. 10.4). This implies that acicular ferrite grows supersaturated
with carbon, but the excess carbon is shortly afterwards rejected into the
remaining austenite.
Acicular ferrite is intragranularly nucleated bainite so it should be possible
to switch between these two morphologies by controlling the nucleation site. A
bainitic microstructure can be replaced by one containing acicular ferrite by
increasing the oxygen, and hence the inclusion content (Ito et al:, 1982). After
all, the appearance of the acicular ferrite microstructure is only different from
Bainite in Steels
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 240 237-276
240
Fig. 10.3 Nomarski interference contrast micrograph illustrating the displace-
ments associated with the formation of acicular ferrite (Strangwood and
Bhadeshia, 1987).
that of bainite because it nucleates intragranularly in steels containing a greater
number density of inclusions than austenite grain surface nucleation sites
(Yang and Bhadeshia, 1986). Acicular ferrite does not grow in sheaves because
their development is sti¯ed by impingement between plates nucleated inde-
pendently at adjacent sites. Indeed, both microstructures can be obtained
under identical isothermal transformation conditions in the same inclusion-
containing steel. Bainite forms when the austenite grain size is small because
nucleation then predominates at the grain boundaries. Subsequent growth then
swamps the interiors of the austenite grains, preventing the development of
acicular ferrite. When the austenite grain size is large, the number density of
inclusions becomes large relative to boundary nucleation sites promoting the
formation of acicular ferrite at the expense of bainite (Fig. 10.5).
This basic theory explains many observations on welds where the heat due
to welds produces a gradient of austenite grain size in the heat affected zone
(HAZ), with the largest grains adjacent to the fusion surface. When steels
containing appropriate inclusions are welded, the ratio of acicular ferrite to
bainite is the highest in the HAZ nearest the fusion boundary where the
austenite grain size is at a maximum (Fig. 10.6a). In the absence of inclusions,
the acicular ferrite content is always very small, Fig. 10.6b (e.g. Imagumbai
et al:, 1985).
In another supporting experiment, Harrison and Farrar (1981) removed the
inclusions by vacuum remelting a weld; when cooled, the steel transformed
into bainite instead of the original acicular ferrite microstructure.
Acicular Ferrite
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 241 237-276
241
Fig. 10.4 Data from experiments in which the austenite is transformed iso-
thermally to acicular ferrite, showing that the reaction stops when the carbon
concentration of the austenite reaches the T
0
0
curve (Strangwood and Bhadeshia,
1987).
We have emphasised that the transformation mechanism for acicular ferrite
is identical to that for bainite. However, all phases can nucleate on inclusions,
including Widmansta
È
tten ferrite (Dube
Â
, 1948; Ali and Bhadeshia, 1991).
Thewlis et al: (1997) have argued that in some welds the so-called acicular
Bainite in Steels
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 242 237-276
242
Fig. 10.5 The effect of austenite grain size on the development of microstructure in
an inclusion-containing steel. A small grain sized sample has a relatively large
number density of grain boundary nucleation sites so bainite dominates the
microstructure, whereas a relatively large number density of intragranular
nucleation sites leads to a microstructure consisting predominantly of acicular
ferrite.
Fig. 10.6 Changes in the microstructure of the heat affected zone of welds, as a
function of the heat input during welding: (a) steel containing titanium oxide
particles; (b) ordinary steel without inclusion inoculation (after Chijiiwa et al:,
1988).
ferrite may predominantly be intragranularly nucleated Widmansta
È
tten ferrite
rather than bainite. They reached this conclusion by noting that the estimated
bainite-start (B
S
) temperature was lower than that at which coarse plates
nucleated on very large inclusions (3±9 mm diameter). Although there is uncer-
tainty in their calculated B
S
values, the conclusion that a mixed microstructure
of intragranularly nucleated Widmansta
È
tten ferrite and intragranularly
nucleated bainite (i.e. acicular ferrite) was obtained seems justi®ed.
Intragranularly nucleated Widmansta
È
tten ferrite can be distinguished readily
from bainite by the scale of the optical microstructure.
Widmansta
È
tten ferrite plates are always much coarser than bainite because
what appears as a single plate using optical microscopy is in fact a pair of self
accommodating plates. The shape deformation consists of two adjacent
invariant-plane strains which mutually accommodate and hence reduce the
strain energy, thus allowing the plates to be coarse (Bhadeshia, 1981a).
Acicular ferrite is sometimes considered to be intragranularly nucleated
Widmansta
È
tten ferrite on the basis of the observation of `steps' at the trans-
formation interface, which are taken to imply a ledge growth mechanism
(Ricks et al:, 1982). The step mechanism of interfacial motion does not
necessarily indicate the mechanism of transformation. The observations are in
any case weak; perturbations of various kinds can always be seen on trans-
formation interfaces between ferrite and austenite. Such perturbations do not,
however, necessarily imply a step mechanism of growth. Evidence that the
residual austenite is enriched in carbon is sometimes quoted in support of
the contention that
a
is Widmansta
È
tten ferrite but as pointed out above, the
enrichment can occur during or after the transformation event.
The weight of the evidence is that the acicular ferrite recognised in most
weld microstructures is intragranularly nucleated bainite. And that the term
acicular ferrite should be reserved for this ®ne microstructure. If coarse
Widmansta
È
tten ferrite forms on inclusions then it can be called `intragranularly
nucleated Widmansta
È
tten ferrite'. The names given to phases are important
because they imply a mechanism of transformation which can be used in
theoretical models. It is particularly important to avoid naming mixtures of
microstructures.
10.3 Mechanism of Nucleation
A popular treatment of acicular ferrite nucleation based on classical hetero-
phase ¯uctuation theory is due to Ricks et al: (1981,1982). It relies on the occur-
rence of chance ¯uctuations in crystal structure. The activation energy (G
)for
a ¯uctuation which is large enough to stimulate critical nucleus depends on the
inverse square of the chemical driving force G
/ G
2
(Chapter 6). With this
theory it is possible to explain why larger spherical non-metallic inclusions are
Acicular Ferrite
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 243 237-276
243
more effective for heterogeneous nucleation. An embryo which forms in
contact with the surface will have a smaller curvature and a corresponding
smaller surface-to-volume ratio when the inclusion is large. A ¯at austenite
grain surface is therefore expected to be a more potent heterogeneous nuclea-
tion site than a spherical inclusion. Furthermore, the energy of the interface
between the ferrite and the inclusion is likely to be larger relative to the case
when ferrite nucleates on austenite grain surfaces. It follows that the activation
energy for nucleation on an inclusion, relative to that for nucleation on an
austenite grain surface should vary as illustrated in Fig. 10.7.
An alternative interpretation uses the bainite nucleation theory discussed in
Chapter 6. Nucleation is said to occur when an appropriate array of disloca-
tions is able to dissociate rapidly. The activation energy is that for the migra-
tion of the embryo/austenite interface; it decreases linearly as the driving force
increases. The driving force must be calculated to allow for the diffusion of
carbon although the overall mechanism of nucleation remains displacive since
the dislocation array considered is glissile.
It follows that the driving force available for nucleation at the highest tem-
perature at which transformation occurs (T
h
) should be proportional to T
h
(Chapter 6). This is found to be the case as illustrated in Fig. 10.8, which is
comparable to Fig. 6.4a. The line is identi®ed with the universal nucleation
Bainite in Steels
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 244 237-276
244
Fig. 10.7 The formation of a truncated spherical nucleus on a spherical inclusion.
The activation energy for nucleation has been normalised with respect to that for
homogeneous nucleation. The calculations assume that the interfacial energy
between austenite and ferrite is the same as that for an austenite grain boundary;
that the inclusion/ferrite and inclusion/austenite interface energies are identical
and that these are both greater than an austenite grain boundary energy. After
Ricks et al: (1981, 1982).
function G
N
that can be used to estimate the acicular ferrite start-temperature
for any alloy.
The experiments shown in Fig. 10.8 included both bainite and acicular fer-
rite, the change in microstructure being achieved by controlling the austenite
grain size. It is evident that both bainite and acicular ferrite can be represented
by the same line, emphasising the conclusion that acicular ferrite is simply
intragranularly nucleated bainite.
The displacive mechanism of nucleation relies on the existence of arrays of
dislocations. It is conceivable that such arrays are generated in the proximity of
non-metallic inclusions due to stresses caused by differential thermal expan-
sion. Such stresses are more dif®cult to accommodate for larger inclusions,
making large inclusions more potent nucleation sites. Arrays of dislocations
are readily found at grain boundaries, accounting for the observation that
austenite grain surfaces are most effective as nucleation sites.
10.4 Nucleation and The Role of Inclusions
Non-metallic inclusions in steels have complex multiphase microstructures
which make controlled experiments designed to reveal nucleation phenomena
rather dif®cult. A popular idea is that the most potent nucleants have a good
lattice match with ferrite. There may then exist reproducible orientation rela-
tionships between inclusions and the ferrite plates that they nucleate (Mills
et al:, 1987). The lattice matching is expressed as a mean percentage planar
mis®t (Bram®tt, 1970). Suppose that the inclusion is faceted on hkl
I
and
Acicular Ferrite
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 245 237-276
245
Fig. 10.8 Plot of the driving force available for the nucleation of bainite or acicular
ferrite at the temperature T
h
, versus T
h
for a series of welding alloys. Note that
each pair of bainite and acicular ferrite points represents a different alloy. After
Rees and Bhadeshia, 1994.
that the ferrite deposits epitaxially with hkl
khkl
I
, with a pair of corre-
sponding rational directions uvw
I
and uvw
inclined at an angle to each
other. The interatomic spacings d along three such directions within the plane
of epitaxy are examined to obtain:
100
3
X
3
j1
jd
I
j
cos d
j
j
d
j
10:1
Data calculated in this manner, for a variety of inclusion phases, are presented
in Table 10.1. A description of the relationship between two crystals with cubic
lattices requires ®ve degrees of freedom, three of which are needed to specify
the relative orientation relationship, and a further two in order to identify the
plane of contact between the two crystals. Mills et al: considered nine different
kinds of epitaxy, con®ned to planes of low crystallographic indices: f001g,
f011g & f111g. The orientation relationships considered are listed in
Table 10.1: the Bain orientation implies f100g
kf100g
I
and < 100>
k
< 011>
I
. The Cube orientation occurs when the cell edges of the two crystals
are parallel.
A comparison with experiments requires not only the right orientation rela-
tionship, but the inclusion must also be faceted on the appropriate plane of
epitaxy. Experiments, however, demonstrate that the ferrite/inclusion orienta-
tion relationship tends to be random (Dowling et al:, 1986). The inclusions,
which form in liquid steel, are randomly orientated but there is a ®xed
a
=
orientation so it follows that the inclusion/ferrite orientation relation must be
random (Fig. 10.9). A contrary view is due to Kluken et al: (1991), who claim
that the -ferrite grains sometimes nucleate on inclusions in the melt. The
Bainite in Steels
[13:36 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-010.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 246 237-276
246
Table 10.1 Mis®t values between different substrates and ferrite. The data are from a
more detailed set published by Mills et al. (1987) and include all cases where the mis®t
is found to be less than 5%. The inclusions all have a cubic-F lattice; the ferrite is body-
centred cubic (cubic-I).
Inclusion Orientation Plane of Epitaxy Mis®t %
TiO Bain {1 0 0} 3.0
TiN Bain {1 0 0} 4.6
-alumina Bain {1 0 0} 3.2
Galaxite Bain {1 0 0} 1.8
CuS Cube {1 1 1} 2.8